The Impact of Feedback in Massive Star Formation. II. Lower Star Formation Efficiency at Lower Metallicity

We conduct a theoretical study of the formation of massive stars over a wide range of metallicities from 1e-5 to 1Zsun and evaluate the star formation efficiencies (SFEs) from prestellar cloud cores taking into account multiple feedback processes. Unlike for simple spherical accretion, in the case of disk accretion feedback processes do not set upper limits on stellar masses. At solar metallicity, launching of magneto-centrifugally-driven outflows is the dominant feedback process to set SFEs, while radiation pressure, which has been regarded to be pivotal, has only minor contribution even in the formation of over-100Msun stars. Photoevaporation becomes significant in over-20Msun star formation at low metallicities of <1e-2Zsun, where dust absorption of ionizing photons is inefficient. We conclude that if initial prestellar core properties are similar, then massive stars are rarer in extremely metal-poor environments of 1e-5 - 1e-3Zsun. Our results give new insight into the high-mass end of the initial mass function and its potential variation with galactic and cosmological environments.Comment: 13 pages, 9 figures, accepted for publication in The Astrophysical Journa

Episodic excursions of low-mass protostars on the Hertzsprung-Russell diagram

Following our recent work devoted to the effect of accretion on the pre-main-sequence evolution of low-mass stars, we perform a detailed analysis of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R) diagram triggered by strong mass accretion bursts typical of FU Orionis-type objects (FUors). These excursions reveal themselves as sharp increases in the stellar total luminosity and/or effective temperature of the protostar and can last from hundreds to a few thousands of years, depending on the burst strength and characteristics of the protostar. During the excursions, low-mass protostars occupy the same part of the H-R diagram as young intermediate-mass protostars in the quiescent phase of accretion. Moreover, the time spent by low-mass protostars in these regions is on average a factor of several longer than that spent by the intermediate-mass stars in quiescence. During the excursions, low-mass protostars pass close to the position of most known FUors in the H-R diagram, but owing to intrinsic ambiguity the model stellar evolutionary tracks are unreliable in determining the FUor properties. We find that the photospheric luminosity in the outburst state may dominate the accretion luminosity already after a few years after the onset of the outburst, meaning that the mass accretion rates of known FUors inferred from the bolometric luminosity may be systematically overestimated, especially in the fading phase.Comment: 15 pages, 12 figure

Toward the Stable Optical Trapping of a Droplet with Counter Laser Beams under Microgravity

To identify the optimum conditions for the optical trapping of a droplet under microgravity, we theoretically analyzed the efficiency of trapping with counter laser beams. We found that the distance between the two foci is an important parameter for obtaining stable trapping conditions. We also performed an optical trapping experiment with counter laser beams under microgravity. The experimental results correspond well to the theoretical prediction

Direct diagnostics of forming massive stars: stellar pulsation and periodic variability of maser sources

The 6.7 GHz methanol maser emission, a tracer of forming massive stars, sometimes shows enigmatic periodic flux variations over several 10-100 days. In this Letter, we propose that this periodic variations could be explained by the pulsation of massive protostars growing under rapid mass accretion with rates of Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the massive protostars have very large radius exceeding 100 Rsun at maximum, and we here study the pulsational stability of such the bloated protostars by way of the linear stability analysis. We show that the protostar becomes pulsationally unstable with various periods of several 10-100 days, depending on different accretion rates. With the fact that the stellar luminosity when the star is pulsationally unstable also depends on the accretion rate, we derive the period-luminosity relation log (L/Lsun) = 4.62 + 0.98log(P/100 day), which is testable with future observations. Our models further show that the radius and mass of the pulsating massive protostar should also depend on the period. It would be possible to infer such protostellar properties and the accretion rate with the observed period. Measuring the maser periods enables a direct diagnosis of the structure of accreting massive protostars, which are deeply embedded in dense gas and inaccessible with other observations.Comment: 5 pages, 3 figures, 1 table, accepted for publication in ApJ

The spin axes orbital alignment of both stars within the eclipsing binary system V1143Cyg using the Rossiter-McLaughlin effect

Context: The Rossiter-McLaughlin (RM) effect, a rotational effect in eclipsing systems, provides unique insight into the relative orientation of stellar spin axes and orbital axes of eclipsing binary systems. Aims: Our aim is to develop a robust method to analyze the RM effect in an eclipsing system with two nearly equally bright components. This gives access to the orientation of the stellar rotation axes and may shed light on questions of binary formation and evolution. Methods: High-resolution spectra have been obtained both out of eclipse and during the primary and secondary eclipses in the V1143Cyg system, using the high-resolution Hamilton Echelle Spectrograph at the Lick Observatory. The Rossiter-McLaughlin effect is analyzed in two ways: (1) by measuring the shift of the line center of gravity during different phases of the eclipses and (2) by analysis of the line shape change of the rotational broadening function during eclipses. Results: The projected axes of both stars are aligned with the orbital spin within the observational uncertainties, with the angle of the primary rotation axis beta_p=0.3+-1.5 deg, and the angle of the secondary rotation axis beta_s=-1.2+-1.6 deg, thereby showing that the remaining difference between the theoretical and observed apsidal motion for this system is not due to a misalignment of the stellar rotation axes. Both methods utilized in this paper work very well, even at times when the broadening profiles of the two stars overlap.[abridged]Comment: Accepted for publication in A&A; 11 pages, 9 figures, 3 tables ; a typo in the abstract has been correcte

Observational signatures of forming young massive clusters: continuum emission from dense HII regions

Young massive clusters (YMCs) are the most massive star clusters forming in nearby galaxies and are thought to be a young analogue to the globular clusters. Understanding the formation process of YMCs leads to looking into very efficient star formation in high-redshift galaxies suggested by recent JWST observations. We investigate possible observational signatures of their formation stage, particularly when the mass of a cluster is increasing via accretion from a natal molecular cloud. To this end, we study the broad-band continuum emission from ionized gas and dust enshrouding YMCs, whose formation is followed by recent radiation-hydrodynamics simulations. We perform post-process radiative transfer calculations using simulation snapshots and find characteristic spectral features at radio and far-infrared frequencies. We show that a striking feature is long-lasting, strong free-free emission from a $\sim$ 10pc-scale HII region with a large emission measure of $\gtrsim 10^7 \mathrm{cm}^{-6} \ \mathrm{pc}$, corresponding to the mean electron density of $\gtrsim 10^3~\mathrm{cm}^{-3}$. There is a turnover feature below $\sim$ 10 GHz, a signature of the optically-thick free-free emission, often found in Galactic ultra-compact HII regions. These features come from the peculiar YMC formation process, where the cluster's gravity effectively traps photoionized gas for a long duration and enables continuous star formation within the cluster. Such large and dense HII regions show distinct distribution on the density-size diagram, apart from the standard sequence of Galactic HII regions. This is consistent with the observational trend inferred for extragalactic HII regions associated with YMCs.Comment: 12 pages, 10 figures, accepted for publication in MNRA

Vortex Reconnection as the Dissipative Scattering of Dipoles

We propose a phenomenological model of vortex tube reconnection at high Reynolds numbers. The basic picture is that squeezed vortex lines, formed by stretching in the region of closest approach between filaments, interact like dipoles (monopole-antimonopole pairs) of a confining electrostatic theory. The probability of dipole creation is found from a canonical ensemble spanned by foldings of the vortex tubes, with temperature parameter estimated from the typical energy variation taking place in the reconnection process. Vortex line reshuffling by viscous diffusion is described in terms of directional transitions of the dipoles. The model is used to fit with reasonable accuracy experimental data established long ago on the symmetric collision of vortex rings. We also study along similar lines the asymmetric case, related to the reconnection of non-parallel vortex tubes.Comment: 8 pages, 3 postscript figure